A wide range of electromechanical devices use electrical motors during operation. One common type of electrical motor is a permanent magnet direct current (DC) motor, also referred to as a PMDC motor. A PMDC motor typically includes two permanent magnets, such as a neodymium or ferrite magnets, with three or more rotors positioned between the magnets. A wire coil, referred to as a brush, in each of the rotors receives an electric current through a commutator and generates an electromagnetic field that is misaligned with the magnetic field of the permanent magnets. The rotors rotate around an axle towards the alignment of the magnetic field, but prior to reaching full alignment with the permanent magnets, the commutator rotates to a position that reverses the electric current and corresponding electromagnetic field in the rotors. The continuous flow of electric current and misaligned magnetic fields generates a rotational motion and torque that drive an axle in the electric motor. In one common alternative design referred to as a brushless DC motor, the permanent magnets rotate inside of an armature coil, and an electronic commutator controller reverses the electromagnetic field in the armature coil to drive the rotating permanent magnets and rotate the axle. In these and other PMDC embodiments, the rotating axle generates a drive torque and provides motive force to a wide variety of mechanical devices.
One class of devices that use PMDC motors includes printers and other imaging devices such as copiers, scanners, facsimile machines, and multi-function devices. Printers can use one or more electric motors, also referred to as actuators, to move paper sheets and other print media through the printer. Many printer embodiments use an electric motor to rotate a cylindrical drum or an endless belt as part of an imaging process. For example, in xerographic printers an electric motor rotates a fuser roller that applies pressure and heat to fix a toner pattern to a print medium. Another example includes indirect or offset inkjet printers. In an indirect inkjet printer, an electric motor rotates an indirect image receiving member, such as a cylindrical drum or an endless belt, past one or more printheads. The printheads eject ink drops onto the indirect image receiving member to form an ink image. The ink image is subsequently transferred to a print medium such as a paper sheet using a “transfix” operation that applies pressure and optionally heat to transfer the ink image to the print medium.
In operation, the temperature of a PMDC motor affects the maximum torque that the axle generates for a given level of electric current and voltage supplied to the motor. The temperature of the motor rises during operation due to the electrical resistance of the motor, mechanical friction, and due to an elevated temperature inside of the printer. As the temperature of the motor rises, the magnetic field of the permanent magnets weakens, the electrical resistances of the windings in the motor increase, and the torque generated at the axle decreases. If the motor temperature is too high, the motor may be unable to provide sufficient torque to operate printer components within specified tolerances, and excessive temperatures can result in damage to the motor. In a printer, a reduction in torque generated by an electric motor can result in a paper jam or other failure in the print process. Thus, monitoring and controlling the temperature of one or more motors in a printer or other mechanical device to prevent overheating enables the device to operate as designed and lengthens the operational life of the device.
While thermistors and other temperature sensors can monitor the temperature of a motor, the temperature sensors add cost and complexity to the printer and can generate unreliable readings. Additionally, fans and other cooling devices also add complexity to the printer and can fail during operation, resulting in an overheated motor. Consequently, improved operations in a printer that prevent overheating of the motors without the need for additional temperature sensors or cooling devices would be beneficial.